This invention relates to inverter circuits and, more specifically, to inverter circuits using controlled rectifiers. It is particularly concerned with a trigger circuit to enable conduction in the controlled rectifiers while preventing mistriggering from spurious signals.
A silicon controlled rectifier (SCR) is basically a diode which operates only when an appropriate trigger signal is applied to its control electrode or gate electrode. The application of an appropriate trigger signal to the gate of a forward biased SCR enables it to conduct current as a diode. In order to permit conduction in a particular SCR, the trigger signal must have certain defined signal characteristics as to magnitude and duration. These characteristics of magnitude and duration, however, are interrelated in that the signal magnitude can be increased thereby decreasing the required signal duration and vice versa.
The interrelated nature of these trigger signal characteristics can result in misfires. That is, an extraneous spurious signal due to external interference can operate to enable conduction in the SCR even though the spurious signal characteristics differ significantly from the normal trigger signal. This spurious signal may be only of short duration but have an extremely high transient signal magnitude or, conversely, may have a very low signal magnitude and a long duration. Because of the interrelations characteristics of a trigger signal required to fire an SCR, a spurious signal can cause the SCR to fire and cause circuit damage.
The typical inverter circuit usually includes two SCRs which are fired or enabled to conduct in an alternate fashion. In order to achieve symmetry in the signals transmitted by the alternately conducting SCRs, a common timing circuit is generally utilized to supply the timing function to the pulse generation circuit of each SCR. Because the timing circuit is common to both pulse generation circuits, spurious signals can be coupled from one pulse generation circuit to another. If both SCRs of an inverter conduct simultaneously, a switchthrough condition occurs wherein a portion of the inverter circuit is short-circuited resulting in very high currents. These high currents can damage the SCR switching devices and shorten the life of the inverter circuit.